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This biotope usually occurs on flats of medium fine sand and muddy sand, most often on the lower shore but sometimes also on waterlogged mid shores. The sand may contain a proportion of shell fragments or gravel. Lan can also occur on the lower part of predominantly rocky or boulder shores, where patches of sand or muddy sand occur between scattered boulders, cobbles and pebbles. Conditions may be tide-swept, and the sediment may be mobile, but the biotope usually occurs in areas sheltered from strong wave action. The sediment supports dense populations of the sand mason Lanice conchilega. Other polychaetes present are tolerant of sand scour or mobility of the sediment surface layers and include the polychaetes Phyllodoce mucosa, Eumida sanguinea, Nephtys hombergii, Scoloplos armiger, Aricidea (Aricidea) minuta, Tharyx spp. and Pygospio elegans. The mud shrimp Corophium arenarium and the cockle Cerastoderma edule may be abundant. The baltic tellin Macoma balthica may be present. On boulder shores, and where pebbles and cobbles are mixed in with lower shore tide-swept sand with dense L. conchilega between the cobbles, the infaunal component is rarely sampled. The infaunal community under these circumstances, provided that the cobbles are not packed very close together, is likely to be similar to that in areas without the coarse material.

Extensive clean fine sand or muddy sand shores with abundant cockles Cerastoderma edule. The community consists of the polychaetes Eteone longa, Scoloplos armiger, Pygospio elegans, Spio filicornis and Capitella capitata, the crustaceans Bathyporeia sarsi, Bodotria arenosa arenosa and Crangon crangon, the spire shell Peringia ulvae, as well as the cockle C. edule and the baltic tellin Macoma balthica. This biotope carries commercially viable stocks of C. edule, and it is therefore possible to find areas of this habitat where the infauna may have been changed through recent cockle dredging. Cockle dredging can result in a reduced bivalve abundance and reduced densities of some polychaete species, including P. elegans (Moore, 1991). At the outer edges of large flats, there may be a zone between the cockle beds and more exposed sands, where there are fewer cockles and B. sarsi is the commoner species.

Fine to very fine muddy sand on the mid shore at the lower extreme of estuaries, and in moderately exposed and sheltered bays and marine inlets, sometimes subject to variable salinity. The infauna is characterised by the polychaetes Eteone longa, Hediste diversicolor (ragworm) and Pygospio elegans, oligochaetes (mostly Tubificoides benedii and T. pseudogaster), the crustaceans Corophium volutator and Crangon crangon, the spire shell Peringia ulvae and the baltic tellin Macoma balthica. The cockle Cerastoderma edule may be abundant, and the sand gaper Mya arenaria may be superabundant, though these species are not always present, or may be missed in core samples due to their large size. The polychaetes Arenicola marina, Polydora cornuta and Capitella capitata, the shrimp Crangon crangon, and the Mussel Mytilus edulis are sometimes present.

Wave-sheltered, mainly upper and mid shore flats of medium to fine sand, often muddy sand. The salinity, although predominantly recorded as variable, probably varies little from fully marine in these broad estuaries. The infauna is characterised by the amphipods Bathyporeia pilosa, Corophium arenarium and C. volutator, and the spire shell Peringia ulvae. Polychaetes and bivalves are limited in their abundance and variety, though the Baltic tellin Macoma balthica may occur. Tidal streams may be strong during spring tides, accounting for the presence of amphipods B. pilosa that are more commonly associated with open coast sandflats.

Muddy sand or fine sand, often occurring as extensive intertidal flats both on open coasts and in marine inlets. The sediment is often compacted, with a rippled surface, areas of standing water, and generally remains water-saturated during low water. Scattered stones, cobbles and boulders with attached fucoids may be present. An anoxic layer is usually present within 5cm of the sediment surface and is often visible in worm casts. The habitat may be subject to variable salinity conditions in marine inlets. The species assemblage is characterised by the lugworm Arenicola marina and the Baltic tellin Macoma balthica. The polychaetes Scoloplos armiger and Pygospio elegans are typically superabundant and common, respectively. Oligochaetes, probably mainly Tubificoides benedii and T. pseudogaster, may be common, and the cockle Cerastoderma edule may be abundant.

Muddy sand or fine sand, often occurring as extensive intertidal flats on open coasts and in marine inlets. The sediment generally remains water-saturated during low water. The habitat may be subject to variable salinity conditions in marine inlets. An anoxic layer may be present below 5 cm of the sediment surface, sometimes seen in the worm casts on the surface. The infauna consists of a diverse range of amphipods, polychaetes, bivalves and gastropods.